Comparison type densitometer



A ril 29, 1952 M. H. SWEET COMPARISON TYPE DENSITOMETER Original FiledApril 16, 1946 3 Sheets-Sheet 1 FIG. 3

INVENTOR 510N506 H. $wr Br 916/;

ATTORNEYS April 9, 1952 M. H. SWEET 2,594,514

COMPARISON TYPE DENSITOMETER Original Filed April 16, 1946 3Sheets-Sheet 2 AMPL lF/ER INVENTOR MONROE H. SWEET ATTORNEYS April 29,1952 M. H. swirl-:1- 2,59

COMPARISON TYPE DENSITOMETER Original Filed April 16, 1946 3Sheets-Sheet 3 INVENTOR MONROE H. SWEET l atenied Apr. 29, 1952 UNITEDSTATES PATENT OFFICE General Aniline & Film Corporation, New York, N.Y., a corporation of Delaware Original application April 16, 1946,Serial No. 662,529, now Patent No. 2,561,243, dated July 1'7,

Divided and this application December 20, 1949, Serial No. 134,095

8 Claims.

This invention relates to null type electronic measuring; circuits, andmore particularly to comparison type densitometers in which the lightincident upon light responsive means from primary and comparison lampsources is balanced to ascertain the value of a characteristic of asample to be measured. This application is a division of co-pendingapplication Serial No. 562,529, filed April 16, 1946, now Patent No.2,561,243, issued July 17, 1951, for Comparison Type Densitoineter andElectronic Measuring Circuit Therelor.

Various kinds of comparison type densitometers have previously beensuggested. As is known to those skilled in the art, the density of asample, such as a piece of photographic film, is a logarithmic functionof its light transmission properties. Accordingly, in comparison typedensitometers the illumination incident upon the photo-responsive meansfrom the comparison lamp must be logarithmically compensated in orderthat the density be indicated upon a uniformly graduated scale.Hitherto, complicated expensive means have been used for suchlogarithmic compensation. Such means have included devices such asvariable area masks, optical wedges and so forth. Other expedients haverelied upon the inverse square law, by moving the comparison lightsource toward and away from the photo-responsive means. All of theseprior devices have been subject to disadvantages due to theirinstability with respect to time. temcrature and humidity, theirinconveniently large dimensions to obtain an adequate range ofmeasurements, and their expense of construction in order to obtain anydegree or accuracy and mechanical and optical quality.

It is among the objects of the present invention to provide a null typemeasuring system employing primary and comparison sources of radiantenergy and electrical means for logarithmically compensating theintensity of the comparison source to obtain a balance of the :energyincident from both sources upon an energy sensitive measuring element;to provide a null type or comparison densitometer in which theillumination intensity of a comparison lamp is logarithmicallycompensated by electrical means, including means effective to indicate,on

a uniformly graduated scale, the density of a sample; to provide a nulltype or comparison densitometer including incandescent lamps as theprimary and comparison light sources and resistance means for varyingthe illumination of the comparison lamp; to provide such a circuit 7includin a single photoemlssive vacuum tube,

means to alternately direct onto said tube light from the comparisonsource and light from the primary source, and mechanism synchronizedwith such means for alternately connecting the output of the tube to apair of input terminals of a measuring circuit; and to provide a simple,inexpensive highly accurate and compact null type or comparison densitoi-eter.

These and other ijects, advantages and novel features of the inventionwll be apparent from the following description and the accompanyingdrawings. In the drawings:

Figure 1 is a schematic wiring diagram of one embodiment of theinvention.

Figure 2 is a schematic wiring diagram of another embodiment of theinvention.

Figure 3 is a set of curves illustrating the relation between sampledensity and electrical characteristics of a comparison source of light.

Figure 4 is a schematic diagram of a further embodiment of theinvention.

Figure 5 is a view on the line 5-5 of Figure 4.

Figure 6 is a schematic wiring diagram of the embodiment of theinvention shown in Figure 4.

Figure "I is a schematic wiring diagram corresponding to a portion ofFigure 6 and illustrating a modified form of the invention.

Figure 8 is a schematic wirin diagram of a further embodiment of theinvention.

Figure 9 is a schematic view, diagrammatically illustrating theapplication of the invention to the measurement of reflection densities.

The present invention is based upon the principle that the candle poweror light output of an incandescent lamp is substantially a logarithmicfunction of the external resistance through which filament currentpasses included in the lamp circuit. Therefore, if such an adjustable orvariable resistance included in the lamp circuit is associated withindicating means having a substantially uniformly graduated scalecalibrated in density, the resistance may be used to vary theillumination of the comparison lamp to obtain a balance between thelight incident upon photoresponsive means directly from the comparisonlamp and that incident upon the photo-responsive means from a primarylamp through an interposed sample. According to the present invention,the resistance may be manually adjusted to obtain such balance or theresistance may be automatically varied in accordance with variations inthe amount of light incident upon the photo-responsive means from theprimary lamp. In the latter instance, a current measuring meter having avirtually uniformly graduated scale may be connected in the comparisonlamp input circuit to indicate directly the density of the sample.

Referring to Figure 1, which illustrates one embodiment which theinvention ma assume in practice, a primary light source and a comparisonlight source ii are arranged to direct light upon photoernissive vacuumtubes 20 and 25, respectively. Light sources l0 and I5 are de-. sirablyincandescent tungsten filament lamps. The light from primary lamp i0 iscondensed by a lens H and directed through a sample 12 mounted on asupport I3 upon the phototube 20. Light from comparison source I5 iscondensed by a lens l4 and directed toward phototube 25, Null indicatingmeans are associated with photqtubes and to indicate a balance betweenthe light beams incident thereupon from their respective associatedlamps it) and I5.

The operating potentials for the circuit ele ments are derived from asuitable source, such as alternating current, connected to terminals I6, #1. Such operating potentials are applied to a voltage doubler tube39 of a, conventional type and the output of tube as is applied to theprimary winding 2! of a transformer 35, Secondary winding 22 oftransformer is connected to the input circuits of lamps lo and i5 in amanner described more fully hereinafter.

A conductor 23, provided with shielding 24, connects cathode 38 ofphototube 26 to anode 2'1 of phototube 25. A conductor 28, likewiseshielded, connects conductor 23 to the control grid 3! of an amplifiertube 40. Cathode 32 of tube 43 is connected to shielding 24 and, througha resistor 33, to cathode 34 of phototube 25. A conventional, nullindicating electronic tube 35, which may be of the cathode ray type usedas a tuning indicator in radio sets, is connected in the output circuitof amplifier tube 48 in the usual manner to indicate a balance betweenthe g1 currents or voltages of phototubes 20. 1 41 25 As the operationof such cathode ray tubes and their circuit connections are well-knownto those skilled in the art, detailed description is not believednecessary. The operating potential applied to amplifier tube and cathoderay? tube are derived from voltage doubler tube 30 through a suitablefilter circuit including inductance 36 and condenserst'l, 38 and 39.

The illumination of primary lamp [0 is controlled by a. variable,resistor 4| connected: in series between one terminal of secondaryWinding 22 of transformer 35 and one terminal of incandescent lamp, ill.The other terminal of lamp Ill is connected by conductors and M. to theother terminal of secondary winding 22. P0- tentiometer 4| is utilizedto control thecurrent and thereby adjust the illumination intensity oflamp ID to the desired value.

The illumination, intensity of comparison lamp I5 is similarly varied.One terminal of lamp I5 is connected through conductor 42 tothe sourceof current comprising the winding 22. Theother terminal thereof isconnected to terminal 41. of adjustable resistance 56. The otherterminal of resistor 5s and its adjustable contact d8lare con nected tothe winding 22 by means of conductor 44. The variable resistor isprovided with uniformly spaced indicia 5i cooperable-with contact S8 forindicating the density of sample l2. These density value indicia mayrange from 0.0 at terminal 41 to 3.0 at terminal 46. This range issufiiciently broad practice to accommodate all ordinarydensity-measurements.

The t, operation of the embodiment show-n in ti Figure 1 is as follows:When the apparatus has been turned on and allowed to warm up, rider 7 48is set at 0.0 density, bringing comparison lamp iii to its maximumbrightness value. Resistance 5 is then adjusted to vary the brightnessof primary lamp iii to balance .the current from phototubes and asindicated by the electronic null indicator :25. Such balancing is donewith sample 52 removed from the path of light from lamp i s to phototube28.

Sample E2 is then interposed between lamp l0 and phototube 2i} and rider4 8 is adjusted until the phototube currents are again balanced asindicated by tube 35. The density of sample I! is then read by notingthe position of arm 43 relative to indicia 5!. In the initialadjustment, tube :35 is not necessarily tuned to a minimum shadow anglebut need only be tuned to a point within its operatin range at whichtube 55 is sensitive to further changes in the current from eitherphototube. This particular shadow angle then reproduced both for thezero setting and for the actual density reading.

The theory will be clear by reference to Figure 3, wherein curves 52,53, 5% represent respectively the variation in the voltage, current andresistance of incandescent lamps i8 and [5 with respect to density ofsample i2. It will be noted from curve 5 that the effective resistanceof the incandescent lamp filament varies substantially directly as thedensity of sample !2. Thus, re.- sistance 5d, which controls theillumination of lamp 55 may be provided with a uni' ormly graduatedscale, including the indicia 51', to indicate directly the density ofsample l2, The circuit thus provides an effective null type comparisondensitometer which is compact, relies upon simple means for effectingbalance between primary and comparison lamps and has good accuracy. Whenan extremely sensitive receiving element-null detector system is used,such as a photomultiplier tube, or when further amplification isprovided, the circuit may be used to read very high neutral or colorfilm densities.

The principles of the circuit of; Figure L may be applied equally to acomparison, type densitometer utilizing barrier layer photocells, asshown in Figure 2. In this figure, elements identical with those inFigure l have, been given corresponding reference characters. Apotential from a suitable source is applied to. terminals 5.6 and 5?connected to primary winding 2! of transformer 35. Secondary winding 22provides the operating potentials for primary lamp l0 and comparisonlamp[5. The lamp controlling circuits are thus the same as thoseoi Figure 1.

Light from primary lamp i5 iscondensed by lens II and directed throughsample 2;mounte d on support it, upon a barrier layer photocell 55.Similarly, light from, a comparison, lamp [5V is condensed by lens Hiand directed uponbarrier layer photocell til. Theoutputs of;photocellsti and 60 are impressed in oppositionacross anindicating meterit, which may be either a-voltmeter, an amrneter or a galvanometer, toindicate a balance of the outputs of photocells 5,5,and 60. The circuitof Figure 2 operates in-the samemanner as that of Figure l. Resistor501s adjusted until meter ill indicates a balance of the outputs ofphotocells 55, and 60. The density of sample 82 then may be readdirectly by noting theposition of pointer 58 with respect to indicia 5|.In the arrangement of Figure 2, no warm-up? period isnecessary,

Figure. 4; illustrates; a. comparison. type: densitometer in which theillumination of the comparison lamp is automatically maintained at sucha level that there is a substantial balance of the light incident uponthe photo-responsive means from the comparison lamp and from the primarylamp. As schematically shown, the arrangement comprises a primaryincandescent lamp I5 and a comparison incandescent lamp 80. A source ofpotential is applied to terminals II, 12, connected, through a switchI3, to conductors 14, 70. Primary lamp I5 and amplifier 85 are suppliedwith energy from the same alternating current source, being connected.directly across conductors I4, I6. Comparison lamp 80 is connected tothe output of amplifier 85 through a pair of conductors 17, I8, and anammeter 90 is connected in series circuit relation with lamp 80 toindicate the intensity of illumination in terms or filament.

The arrangement of Figure 4 includes a chopper mirror 95 rotated by amotor 8|. A single pole, double-throw switch I is operated insynchronism with mirror 95 by motor 8I, as through the medium of driveshaft 82. Switch I00 is connected to amplifier 85 by conductors 83, 84,and motor 8i is connected to conductors "I4, 76 through a switch 86.Referring to Figure 5, which is a face view of mirror 95, the mirrorcomprises a glass disk having alternate sectors 81 silvered on theirback surfaces to provide re fleeting surfaces, and intermediate sectors88 left clear to pass light therethrough.

Mirror 95 is arranged at an angle with respect to lamps I5 and 80 and aphototube I05 havin its output circuit connected to amplifier 85. Lightfrom primary lamp I5 is condensed by a lens BI and directed, through asample 92 mounted on a support 93, onto mirror 95. Comparison lamp 80 isaligned directly with phototube I05 and its light is condensed by a lens94 and directed upon the phototube through mirror 95. As the mirror isrevolved, light from lamp I5 and light from lamp 80 will be alternatelyreflected or transmitted onto phototube I05. The light from lamp I5 isreflected by mirrored sections 81 on to the phototube whereas the lightfrom comparison lamp 00 passes directly through clear sections 88 on tothe phototube. As will be described in connection with Figure 6, switchI00 alternately connects different input terminals of amplifier 85 tothe output circuit of phototube I05. The circuit connections are suchthat the output current of amplifier 85 is varied, varying theillumination of comparison lamp 80 and this output current is indicatedby meter 90. The amplifier in effect provides a resistance in seriescircuit relation with comparison lamp 80 and thus varies the filamentcurrent directly and due to the inherent characteristic of the lamp alsothe intensity logarithmically with respect to variations in the outputimpedance of amplifier 85, so that meter 90 is effective to indicate thefilament current in terms of density of sample 92 upon a uniformlygraduated scale.

Figure 6 is a schematic wiring diagram illustrating the operation of thearrangement shown in Figure 4. As shown, phototube I05 may comprise aphotomultiplier tube having a cathode 96, an anode 91 and multiplierelements or dynodes 98. The operating potentials for multiplier tube I05are derived from a potentiometer or other voltage arrangement IOI havingone terminal I02 connected to the negative termlnalof a suitable sourceof substantially constant direct current potential. The other terminalII G -of potentiometer IN is connected, in series with an electronic,tube IIO, to the other terminal of the source of direct currentpotential. Terminal I02 is connected to cathode 90, and equi-spacedpoints on the points on the potentiometer are connected to dynodes 98.The dynode 98 next to anode 97 is connected also to the cathode 03 oftube IIO. A voltage stabilizer means may be connected between anode 01and dynode 98 in the manner described and illustrated in my co-pendingapplication Serial No. 647,932, if deemed necessary.

Cathode I03 is connected to a terminal of a grid biasing source ofpotential, such as a battery I04, and the opposite terminal of batteryI04 is connected to screen grid I08 and, through a grid biasing resistorIN, to a junction point I08. Junction point I08 is connected to anode 91and to control grid I I I of tube I I0.

The operation of the circuit thus far described is the same as describedin my co-pending applications Serial Nos. 570,627, now Patent No.2,478,163, issued August 2, 1949, and 647,932, now Patent No. 2,457,747,issued December 28, 1948. Linear attenuation of the operating potentialsapplied to multiplier tube I05 is effected by the operation of tube I I0in such a manner that the output current of tube I05 varies inversely asa logarithmic function of the anode current of multiplier tube I05. Theanode current of tube I05 varies directly as the amount of illuminationincident upon its cathode 96. Consequently, the output current of tube II0 is an inverse logarithmic function of the amount of light incidentupon cathode 98 of tube I05. This incident light is a direct function ofthe transmission of sample 92 and a logarithmic function of the densityof the sample. Accordingly, the output current of tube H0 is a directmeasure of the density of sample 92.

As anode 97 current increases, with increased incident illumination, thepotential drop across biasing resistor I07 increases and thus controlgrid III becomes more negative. This decreases the output current oftube H0 and accordingly decreases the operating potentials applied bybleeder resistor or potentiometer IOI to the elements of multiplier tubeI05. A corresponding action, in reverse direction, occurs when theillumination on tube I05 decreases.

The output potential drop across potentiometer ml is applied to theinput circuit of amplifier 85. For this purpose, anode I I2 of amplifier85 is connected to the positive terminal II3 of a suitable source ofsubstantially constant potential. Negative terminal II4 of the urse isconnected, in

series circuit relation With comparison lamp ceived by tube I05 throughclear sector 88, contact II'I engages fixed contact I25. Contact I20 isconnected to the cathode I 2I of amplifier and contact I25 is connectedto the control grid I26 of amplifier 85. Variable condensers I22, I23connect contacts I20, I25, respectively, to conduc tor I24 connected tonegative terminal I02 of potentiometer or bleeder resistor IOI. A thirdvariable condenser I 21 is connected across the input circuit ofamplifier 85 and serves to stabilize the grid-cathode voltage during theoperation of the circuit.

The response of phototube I05 to the differing lights directed thereuponfrom lamps I5 and 80, is sufiiciently rapid that the voltage drop acrossbleeder resistor I accurately reflects the logarithmic intensity of eachof the lamps during the time that light from either of the lamps isreceived by phototube I05. This potential drop is applied alternately tocathode I2I and grid I26 of amplifier 85. If the comparison and primarybeams are of the same intensity, the voltage drops across condensers I22and I23 will be identical and therefore terminals I25 and I20 will be atthe same potential.

Assume that the light from primary lamp I5 is momentarily interrupted bythe insertion of sample 92. The polarity relations are such thatconsubstantially equal. The variation in the illumidenser I2 2 will becharged to a lower voltage than condenser I23. Consequently, grid I26willbecome more negative with respect to cathode I2I. This, in turn,reduces the output current of amplifier 85 and thereby reduces theintensity of comparison lamp 80. Electrical equilibrium will followoptical equilibrium, and the circuit will stabilize at the point wherelight falling on phototube I05 from each of the lamps I5 and 80 isnation of lamp 80 is measured, in terms of its current, by meter 90which thus gives a direct indication, on a substantiallyuniformlygraduated scale, of the density of sample 92.

The use of the logarithmically responsive circuit associated withphototube I05 provides a wide density range over which the sensitivityof the circuit is approximately constant. If a linearly responsivecircuit were used, the indicating system would be 1000 times assensitiveat densities of sample 92 in the vicinity of 0 as at densitiesin the neighborhood of' 3.0 resulting in serious circuit complications.

The described arrangement provides an optical electronic system whereinthe density of a sample is measured by means of an optical feedbackcircuit in which the primary and comparison lamp beams are alternatelydirected at a single phototube and the electronic circuit is so arrangedthat the intensity of the comparison l.-

beam is automatically stabilized at such a level as to produce virtuallyno flicker in the light incident upon the phototube. By varying theresistance included in the circuit of the comparison lamp, a directreading of density is obtainable due to the logarithmic relation betweensample density and comparison lamp resistance. I

Figure 7 illustrates a modified form of a photomultiplier tubeembodiment of the invention shown in Figure 6. In Figure 7, a motoroperated rheostat is used to vary the brightness of the comparison lampautomatically to achieve a balance of the light alternately incidentupon the photomultiplier tube I05 of Figure 6, from the primary andcomparison lamps. The portion of Figure '7 to the left of the dot anddash line is the same as the corresponding portion of" Figure 6, andsimilar reference numerals have been used to designate identicalelements. The embodiment of Figure '7 is. used with the chopper mirrorarrangement of Figures 4 and 5, as indicatedby the broken line 82representing the mirror axis connected to switch I00.

,In this embodiment of the invention, a voltage stabilizer means, such'as a voltage stabilizer tube l mirror 95.

put current of amplifier tube 85.

I30, is connected between terminal I3I of resistor series I0! andcathode I 03 of tube I I 0, in the same manner as described andillustrated in my said cop'en'ding application Serial No. 547,932, nowPatentNo. 2,457,747, issued December 28, 1948. As explained in saidcopending application, stabilizer tube I maintains a substantiallyconstant, relatively high voltage between anode 21 and the last dynode95 of phototube M5 to stabilize operation of the circuit. The remainderof the circuit, as far as amplifier 05 operates in the same manner asdoes the circuit of Figure 6.

However, in the embodiment of Figure 7, amplitrolling the illuminationof lamp 80 from a source of potential connected to terminals I36.Rheostat I55 is provided with density indicia- I37 in the same manner asin rheostat of Figures 1 and 2. Lamp 30 is connected in series betweenone terminal 36 and one terminal I 33 of rheostat I35, and the movablecontact I00 of the rheostat is connected to the other terminal I35.

Movable contact 55 is operated in a suitable manner by connection,either directly or through suitable reduction gearing, to the armatureshaft I ll of a motor 545 indicated as a shunt motor having an armatureMS and a shunt field winding I 47. When the densitometer is energized, apotential is applied constantly to armature I 46. However, the polarityand energization of field I4! is controlled by polarized relay I50having a'pair of pivotally mounted armatures I5I, I52 mechanicallyinterconnected as indicated by the broken line 53. Armature I5I isconnected to relatively positive terminal I I 3 and armature I 52 isconnected to relatively negative terminal II l. Armature I5I may engageeither one of a pair of' contacts I55, l55, and armature I52 may engageeither one of a pair of contacts I56, I51. Contacts I54 and I5"! areinterconnected to each other and to one end of shunt field winding I41.Contacts I55 and I55 are interconnected to each other and to the otherterminal of the shunt field winding. Therefore, the relative polarity offield winding I4? will depend upon which direction armatures l5I and IE2are swung by energization'ofrelay #55.

Relay I50 is connected in the output circuit of amplifier tube 55 inparallel circuit relation with a source of biasing potential comprising,for example, a battery I50 in serie circuit relation With apotentiometer NH.

The operation of the described circuit is as follows. With specimen 92removed from the path of light extending from primary lamp I5 to mirror95 (Figure 4), potentiometer I6I is adjusted to such a value.that relayI50 will energize motor I45 to rotate rheostat contact I40 to the 0.0density position, and so that relay I50 will then move itsarmatures I5I,I52 to the neutral position. This assures a maximum brightness ofcomparison lamp 8%.

Sample 92 is then placed between lamp I5 and The resulting unbalance inthe amount of light reaching phototube I05 from lamps I5 and Bil willefiect a change in the out- This efiect takes place through theinteraction of condensers I22, I23 as controlled by-switch I00 operatingin synchronism with mirror 95, all as described in connection with theoperation in the embodiment of the invention shown in Figure 6. Thebalance between output current of tube 85 and the output current ofbattery I60 is thus upset, and this unbalance thereof will effectoperation of relay I50 to swing its armatures in a direction to energizemotor I45 to rotate arm I40 to adjust the brightness of comparison lamp80 until rebalance of the light incident upon phototube I is efiected.When such balance has been attained, relay I50 again assumes the neutralposition shown in Figure '1 and motor I45 comes to a stop. The densityreading of sample 92 may then be taken by noting the position of arm I40with respect to indicia I31.

Figure 8 illustrates an embodiment of the invention in which only asingle source of light is used to attain a density reading by thecomparison method. In the embodiment shown in Figure 8, a rheostat I10is connected, in series with an incandescent lamp I15, to a source ofpotential connected to terminals I1I. Density indicia I12 are arrangedaround rheostat I10, in the same manner a previously described, forcooperation with the movable contact I13 of the rheostat. Light fromlamp I15 is condensed by a lens I16 and directed through a sample I11mounted on a support I18 onto the cathode I8! of the photoemissivevacuum tube I80.

Tube I80 is connected in a conventional manner to an amplifier tube I85having its cathode I9I connected to the midpoint of the secondarywinding I82 of a transformer I98. The primary winding I83 of thetransformer is connected to terminals I'II. Anode I84 of phototube I80is connected to anode I86 of amplifier tube I85. Cathode IBI of thephototube is connected to the control grid I81 of amplifier I85. Thecontrol grid is also connected, through a parallel connected condenserI88 and resistor I89, to one terminal of secondary winding I82. Theother terminal of winding I82 is connected, in series with an indicatingmeter I95 to anode I86.

The arrangement operates in the following manner. Arm I13 of rheostatI10 is adjusted to the 0.0 density position, with sample I11 removed,thus attaining the maximum brightness of lamp I15. Under theseconditions, the indication of meter I90 is noted. Sample I11 is theninterposed in the path of light from lamp I15 to phototube I88. Arm I13is then adjusted until such time as meter I90 has the same reading as ithad with sample I11 removed and arm I13 at the 0.0 density position. Thedensity of sample I11 is then read by noting the relation of arm I13 toindicia I12.

Figure 9 chematically illustrates how the principles of the inventionmay be applied to the measurement of reflection densities. For thispurpose, the elements of Figure l are illustrated as arranged to readsuch reflection densities. Thus, light from primary lamp I0 which may beeither lamp I0 of Figures 1 and 2, lamp 15 of Figures 4 through '1 orlamp I15 of Figure 8, is

measuring circuits, such as null-type densitometers, are provided inwhich the external resistance included in the energizing circuit ofan'incandescent lamp may be varied either manually or automatically togive a direct reading of density values on a uniformly graduated scale.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the principlesthereof, it will be understood that the invention may be otherwiseembodied without departing from such principles.

I claim:

1. A comparison densitom-eter comprising, in combination, a primaryincandescent lamp; a comparison incandescent lamp; said lamps havingsuch characteristics that their light intensity is logarithmicallyrelated to the electrical energy supplied; a light responsive means;mechanism eifective alternately to direct upon said means light fromsaid primary lamp through a sample whose density is to be measured andlight directly from said comparison lamp; variable impedance means inseries circuit relation with a source of potential and said comparisonlamp; indicating means, having a substantially uniform density scale,operatively connected to said impedance means; means, having a pair ofinput terminals, operative to vary the impedance of said impedance meansas a function of the relative potentials of said input terminals; andswitching means operable in synchronism with said mechanism to connectsaid input terminals alternately to the output of said light responsivemeans to control said variable impedance means to vary the potentialapplied to said comparison lamp to balance the light incident upon saidlight responsive means from both of said lamps.

2. A comparison densitometer comprising, in combination, a primaryincandescent lamp; a comparison incandescent lamp; said lamps havingsuch characteristics that their light intensity is logarithmicallyrelated to the electrical energy supplied; a photomultiplier tube;bleeder resistor means connected to the elements of said tube andefiective to apply operating potentials thereto; an amplifier tubehaving its output connected in series circuit relation with said meansand a source of substantially constant potential; means connecting theinput of said amplifier tube to the output of said multiplier tube,including circuit connections effective to vary the grid bias of saidamplifier tube as an inverse function of the multiplier tube outputcurrent, whereby such operating potentials are varied inversely as themultiplier tube output current; rotating mirror means in operativeassociation with-said lamps and said multiplier tube, and efiectivealternately to refleet light received from said primary lamp, through asample whose density is to be measured, onto said multiplier tube and totransmit light directly from said comparison lamp onto said multipliertube; variable impedance means in series circuit relation with a sourceof potential and said comparison lamp; indicating means, having asubstantially uniform density scale, 0peratively connected to saidimpedance means; means, having a pair of input terminals, operative tovary the impedance of said impedance means as a function of the relativepotentials of said input terminals; and switching means operable insynchronism with said mirror means to connect said input terminalsalternately to the output of said amplifier tube to control saidvariable impedance to vary the potential applied to said comparison lampto balance the light inci- 11 dent upon said photomultiplier tube fromboth oi said lamps.

3. A comparison densitometer comprising, in combination, a primaryincandescent lamp; a comparison incandescent lamp; said lamps havingsuch characteristics that their light intensity is logarithmicallyrelated to the electrical energy supplied; a light responsive means;mechanism effective alternately to direct upon said means light fromsaid primary lamp through a sample Whose density is to be measured andlight directly from said comparison lamp; an indicating meter having asubstantially uniformly graduated density scale; variable impedancemeans in series circuit relation with a source of potential; said meterand said comparison lamp, and having a pair of input terminals; theimpedance of said impedance means varying as a function of the relativepotentials of said input terminals; and switching means operable insynchronism with said mechanism to connect said input terminalsalternately to the output of said light responsive means to control saidvariable impedance means to vary the potential applied to saidcomparison lamp to balance the light incident upon said light responsivemeans from both of said lamps.

4. A comparison densitometer comprising, in combination, a primaryincandescent lamp, a comparison incandescent lamp; said lamps havingsuch characteristics that their light intensity is logarithmicallyrelated to the electrical energy supplied; a photomultiplier tube;voltage means connected to the elements of said tube and efiective toapply operating potentials thereto; a variable impedance means having aninput connected to the output of said tube and an output in seriescircuit relation with said voltage means and a source of substantiallyconstant potential, said variable impedance means controlling thepotential of said voltage means as a function of the tube output currentto correspondingly vary such operating potentials; mechanism effectivealternately to direct upon said tube light from said primary lampthrough a sample Whose density is to be measured and light directly fromsaid comparison lamp; an indicating meter having a substantiallyuniformly graduated density scale; an electronic tube having its outputconnected in series circuit relation with a source of potential,

said meter and said comparison lamp, and having a pair of inputterminals; the output impedance of said electronic tube varying as afunction of the relative potentials of said input terminals; andswitching means operable in synchronism with said mechanism to connectsaid input terminals alternately to the output of said variableimpedance means to control said electronic tube output impedance to varythe potential applied to said comparison lamp to balance the lightincident upon said photomultiplier tube from both of said lamps.

5. A comparison densitometer comprising, in combination, a primaryincandescent lamp; a comparison incandescent lamp; said lamps hav mesuch characteristics that their light intensity is logarithmicallyrelated to the electrical energy supplied; a photomultiplier tube;bleeder resistor means connected to the elements of said tube andeffective to apply operating potentials thereto; an amplifier tubehaving its output connected in series circuit relation with said meansand a source of substantially constant potential; means connecting theinput of said amplifier tube to the output of said multiplier tube,including circuit connections effective to vary the grid bias of saidamplifier tube as an inverse function of the multiplier tube outputcurrent, whereby such operating potentials are varied inversely as themultiplier tube output current; rotating mirror means in operativeassociation with said lamps and said multiplier tube, and effectivealternately to reflect light received from said primary lamp, through asample whose density is to be measured, onto said multiplier tube and totransmit light directly from said comparison lamp onto said multipliertube; an indicating meter having a substantially uniformly graduateddensity scale; an electronic tube having its output connected in seriescircuit relation with a source of potential; said meter and saidcomparisonlamp and having a pair of input terminals; the outputimpedance of said electronic tube varying as a function of the relativepotentials of said input terminals; and switching means operable insynchronism with said mirror means to connect said input terminalsalternately to the output of said multiplier tube to control saidelectronic tube output impedance to vary the potential applied to saidcomparison lamp to balance the light incident upon said photomultipliertube from both of said lamps.

6. A comparison densitometer comprising, in combination, a primaryvincandescent lamp; a comparison incandescent lamp; said lamps havingsuch characteristics that their light intensity is logarithmicallyrelated to the electrical energy supplied; a photomultiplier tube;bleeder resistor means connected to the elements of said tube andeffective to apply operating potentials thereto; an amplifier tubehaving its output connected in series circuit relation with said meansand a source of substantially constant potential; means connecting theinput of said amplifier tube to the output of said multiplier tube,including circuit connections effective to vary the grid bias of saidamplifier tube as an inverse function of the multiplier tube outputcurrent, whereby such operating potentials are varied inversely as themultiplier tube output current; rotating mirror means in operativeassociation with said lamps and said multiplier tube, and efiectivealternately to re fiect light received from said primary lamp, through asample whose density is to be measured, onto said multiplier tube and totransmit light directly from said comparison lamp onto said multipliertube; an indicating meter having a substantially uniformly graduateddensity scale; an electronic tube having its output connected in seriescircuit relation with a source of potential, said meter and saidcomparison lamp; a capacitance connected across the input of saidelectronic tube; a condenser having one terminal connected to thecathode of said electronic tube; means connecting the other condenserterminals to one terminal of said bleeder resistor means; and switchingmeans operable in synchronism with said mirror means to connect said onecondenser terminal alternately to the other terminal of said bleederresistor means to vary the grid bias of said electronic tube meansinversely as a function of the sample density to correspondingly varythe operating potential of said comparison lamp to balance the lightincident upon said multiplier tube from both of said lamps.

7. A comparison densitometer comprising, in combination, a primaryincandescent lamp; a comparison incandescent lamp; said lamps havingsuch characteristics that their light intensity is logarithmicallyrelated to the electrical energy supplied; a photomultiplier tube;voltage means connected to the elements of said tube and efiec tive toapply operating potentials thereto; variable impedance means having aninput connected to the output of said tube and an output in seriescircuit relation with said voltage means and a source of substantiallyconstant potential; said variable impedance means controlling thepotential of said voltage means as a function of the tube output currentto correspondingly vary such operating potentials; mechanism effectivealternately to direct upon said tube light from said primary lampthrough a sample whose density is to be measured and light directly fromsaid comparison lamp; a variable resistor, including a contact armmovable relative to a substantially uniformly graduated density scale,in series circuit relation with said comparison lamp and a source ofelectric potential; said contact arm being adjustable to vary theillumination of said comparison lamp; a reversible electric motoroperative to adjust said contact arm; a polarized relay directionallycontrolling the energization of said motor; an electronic tube having apair of input terminals and having its output circuit connected inseries with a source of potential and said relay; a source of biasingpotential connected in parallel circuit relation with said relay; theoutput impedance of said electronic tube varying as a. function of therelative potentials of said input terminals; and switching meansoperable in synchronism with said mechanism to connect said inputterminals alternately to the output of said variable impedance means tocontrol said electronic tube output impedance to control the polarity ofsaid relay to in turn control the directional energization of said motorto adjust said contact arm to vary the illumination of said comparisonlamp to balance the light incident upon said photomultiplier tube fromboth of said lamps.

8. A comparison densitometer comprising, in combination, a primaryincandescent lamp; a comparison incandescent lamp; said lamps havingsuch characteristics that their light intensity is logarithmicallyrelated to the electrical energy supplied; a photomultiplier tube;bleeder resistor means connected to the elements of said tube andeffective to apply operating potentials thereto; an amplifier tubehaving its output connected in series circuit relation with said meansand a source of substantially constant potential; means connecting theinput of said amplifier tube to the output of said multiplier tube,including circuit connections effective to vary the grid bias of said 14ing potentials are varied inversely as the multiplier tube outputcurrent; rotating mirror means in operative association with said lampsand said multiplier tube, and effective alternately to refiect lightreceived from said primary lamp,

' through a sample whose density is to be measured,

onto said multiplier tube and to transmit light directly from saidcomparison lamp onto said multiplier tube; a variable resistor,including a contact arm movable relative to a substantially uniformlygraduated density scale, in series ciramplifier tube as an inversefunction of the multiplier tube output current, whereby such operatcuitrelation with said comparison lamp and a source of electric potential;said contact arm being adjustable to vary the illumination of saidcomparison lamp; a reversible electric motor operative to adjust saidcontact arm; a polarized relay directionally controlling theenergization of said motor, an electronic tube having a pair of inputterminals and having its output circuit connected in series with asource of potential and said relay; a source of biasing potentialconnected in parallel circuit relation with said relay; the outputimpedance of said electronic tube varying as a function of the relativepotentials of said input terminals; a capacitance connected across theinput of said electronic tube; a condenser having one terminal connectedto the grid of said electronic tube; a condenser having one terminalconnected to the cathode of said electronic tube; means connecting theother condenser terminals to one terminal of said bleeder resistormeans; and switching means operable in synchronism with said mirrormeans to connect said one condenser terminal alternately to the otherterminal of said bleeder resistor means to vary the grid bias of saidelectronic tube inversely as a function of the sample density to controlthe polarity of said relay to in turn control the directionalenergization of said motor to adjust said contact arm to correspondinglyvary the illumination of said comparison lamp to balance the lightincident upon said photomultiplier tube from both of said lamps.

MONROE H. SWEET.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,919,182 Fitzgerald July 18,1933 2,245,034 Harrison July 10, 1941

